It's best to replace every season, especially with vigorous vegetable crops that are used as vegetables crops. The main issue isn't that its spent of nutrients or anything like that, it's that the pine has decayed/collapsed to the point where it now holds some perched water, and doesn't have as much free air space. You can, of course, make a subjective decision based on how compacted the mix appears to you.

I generally use the spent mix to replenish my raised bed, along with other amendments. The 5-1-1 mix is supposed to be affordable, allowing it to be re-used. But nowadays, what is affordable is more debatable than ever.

The 5-1-1 is very cost effective when compared to other pre-bagged mixes.

Since I use uncomposted bark, and I often use turface in lieu of peat moss, my 5-1-1 mix
lasts a good deal longer (though I recognize that my results aren't typical).

If I were re-using 5-1-1, I would run the mix over a 1/16 inch screen (window/door screen)
to remove the peat particles primarily. I would increase the Perlite and increase the bark,
then test the drainage and water-holding capacity to make sure that I'd removed enough of the
fine particulate and added enough fresh, course material.

Generally, 5-1-1 should become a bit finer through time. Fresh 5-1-1 is great for larger pots, but may allow smaller pots to dry out too quickly. A good suggestion is to sterilize used 5-1-1 in an oven at 180F for 30 minutes, then reuse it for seedlings and cuttings in smaller pots since it should now have a finer structure.

The Turface and granite, or in my case pumice doesn't decompose, only the bark breaks down. I just went through my first season, and I don't have raised beds to dump it in, it wasn't that cheap to do all the containers at once, plus it was a lot of time spent sifting. My plan is to let the mix dry out, sift out the broken down bark, toss it into the wheel barrow, mix in a little fresh bark and reuse. The exception would be if a plant had some disease problem, then I would either sterilize or toss it in the yard.